Cement / rock interaction in geothermal wells

One of the main issues associated with the exploitation of geothermal energy is the durability of the cement that is used downhole to cement the steel casing to the formation. Cement durability can have a major impact on the lifetime of geothermal wells, which do not usually last as long as desirable. The cement formulations used in the construction of geothermal wells are designed to provide mechanical support to the metallic well casings and protect them against the downhole harsh environment, which often leads to corrosion. This research is focused on the way that these formulations interact with the surrounding rock formation in geothermal environments, and aims to understand whether these are likely to affect the cement durability and, consequently, the geothermal well lifetime. The experimental work in this thesis consists of examining the changes in the interfacial transition zone (ITZ) that forms between geothermal cements and the volcanic rocks, after hydrothermal treatment. Holes were drilled in blocks of volcanic rocks and cement slurries with distinct formulations were poured into the cavities. The assemblages were autoclaved under typical geothermal conditions. The main variables under study were the cement formulation, the temperature of curing (150°C and 290°C), the presence of drilling mud, CO₂ exposure and the type of rock. The results show that with all the Portland cement based systems a series of chemical reactions occur at the interface between the cement and the rock, the ITZ, where migration of Ca²⁺ and OH⁻ ions occurs from the cement into the rock pores. These reactions are ongoing, which occur faster during the first days/few weeks of curing, mostly driven by physical process of cement movement into the rock, followed by a slower second stage, controlled mostly by chemical driving forces. This work highlights the interdependence between the chemical and physical interactions between geothermal cements and volcanic rocks which are complex. Variables such as temperature and time of curing and silica addition affect the cement phases that form, while the amount of amorphous silica and rock permeability dictate the extent of rock interaction. The presence of carbon dioxide influences the extent of rock/cement interaction and this can be controlled by the rock permeability and cement formulation. Consequently, most of the above mentioned variables were found to have an impact on the geothermal cement durability, which depends on the way these factors are combined.